The document EP 0 588 026 A2 and the thesis entitled “Physikalische Modellbildung von IGCTs für die Schaltungssimulation”, chapter 3 by H. G. Kuhn, Munich, 2002 describe gate turn-off thyristors (GTO) and integrated gate commutated thyristors (IGCT). Such thyristors comprise a four-layer pnpn structure comprising an (n+) doped cathode layer, on which a cathode metallization is arranged, adjacent to a p doped base layer, which is contacted by a gate electrode, followed by an (n−) doped base layer and a (p+) doped anode layer. The anode layer is covered by an anode metallization. When such a GTO or IGCT is switched-off, holes are collected efficiently laterally within the (p−) base layer to the gate. The holes reaching the (n+) cathode region may retrigger the thyristor preventing the semiconductor to switch off. Thereby, the maximum turn-off current is reduced.
The GTO is normally used in snubbered operation, the snubber limiting voltage rise. In this mode of operation all the holes that can reach the cathode and retrigger the device are from stored charge. There distribution is widely determined by the on-state preceding turn-off.
In most IGCT applications snubberless operation is rule, and voltage rise is often only limited by the turn-off device itself. Thereby, the field distribution gets steep enough to generate new carrier pairs during turn-off. This mode is called dynamic avalanche. If an IGCT is driven into this mode, holes generated at turn-off may also reach the cathode, retriggering the device and thereby limiting maximum turn-off current.
A power semiconductor device according to the prior art also comprises a semiconductor device of the above structure, but with a first base layer of modulated conductivity. Such a device is e.g. described in U.S. Pat. No. 4,170,502. In this embodiment the on-state takes advantage of the lower first base layer doping concentration, whereas turn-off is favored by the lower lateral resistance of said layer towards the gate. What is instrumental in improving this type of GTO thyristor is the maximum concentration of said first base layer, which is lower adjacent to the cathode emitter and higher towards the gate contact.